1. Field of the Invention
The present invention relates to an apparatus for producing polycrystalline silicon which deposits polycrystalline silicon on the surface of a heated silicon seed rod to produce a polycrystalline silicon rod.
Priority is claimed on Japanese Patent Application No. 2008-164298, filed Jun. 24, 2008, and Japanese Patent Application No. 2009-135831, filed Jun. 5, 2009, the content of which is incorporated herein by reference.
2. Description of Related Art
An apparatus employing Siemens method is known as an apparatus for producing polycrystalline silicon. In the apparatus for producing polycrystalline silicon using the Siemens method, a number of silicon seed rods are arranged in the reactor. The silicon seed rods in the reactor are heated, and raw material gas including mixed gas of chlorosilane gas and hydrogen gas is supplied to the reactor to come into contact with the heated silicon seed rods. On a surface of a silicon seed rod, polycrystalline silicon is produced by a hydrogen reduction reaction and a thermal decomposition reaction of the raw material gas.
In such an apparatus for producing polycrystalline silicon, the silicon seed rods are disposed so as to stand upright on the electrodes arranged at an inner bottom portion of the reactor. Then, an electric current is applied to the silicon seed rods from the electrodes, and the silicon seed rods generate heat by the resistance thereof. At this time, the raw material gas which is jetted from below is come into contact with the surfaces of the silicon seed rods to form polycrystalline silicon rods. A plurality of the electrodes holding the silicon seed rods is provided so as be distributed over almost the whole region of the inner bottom face of the reactor, and as described in Japanese Patent Unexamined Publication No. 2007-107030, is provided in through-holes of a bottom plate of the reactor so as to surrounded by an annular insulating material.
In the apparatus for producing polycrystalline silicon described above, the gas temperature in the reactor becomes elevated to 500 to 600° C. at a maximum. Thus, the electrode holders holding electrodes are cooled by circulating cooling water in the electrode holders. However, since the insulating material provided between the through-holes of a bottom plate of the reactor and the electrode holders cannot be directly cooled, the shape thereof is apt to be damaged due to the heat in the reactor, which is apt to cause deterioration of the insulating function. In this case, if a ceramic-based insulating material is used, there is a possibility that the insulating material will become damaged, since the insulating material is not be capable of absorbing the thermal expansion difference between the bottom plate of the reactor and the electrode holder.